1. Field of the Invention
The present invention relates to method for making nanoparticles and method for making a thin film, and particularly to a method for making Cu2-xSe nanoparticles with photoelectrical properties and a method for making a deposited Cu2-xSe thin film by electrophoresis.
2. Description of the Prior Arts
Group II-VI and Group II-VI solar cells are named thin-film solar cells since they can be fabricated on flexible substrates, can be manufactured in quantity production and the quantity production is low-cost. The common thin-film solar cells include cadmium telluride (CdTe), as mentioned in Matsumoto, H. et al. Solar cells 11, 367 (1984), Oladeji, I. O. et al. Sol. Energy. Mater. Sol. Cells 61, 203 (2000), copper indium selenide (CIS), as mentioned in Vidyadharan Pillai, P. K. et al. Sol. Energy. Mater. Sol. Cells. 51, 47 (1998), Gordillo, G et al. Sol. Energy. Mater. Sol. Cells. 77, 163 (2003), and copper indium gallium selenide (CIGS), as mentioned in Sakurai, T. et al. Sol. Energy. Mater. Sol. Cells. 95, 227 (2011), Oda, Y. et al. Curr. Appl. Phys. 10, S146 (2010).
CdSe, belonging to group II-VI on the periodic table, is an excellent material for thin-film solar cells. The energy gap of CdSe is 1.4 eV to 1.5 eV, and CdSe is a direct energy gap material. Besides, CdSe has a good absorption coefficient (α) larger than 5×105 cm−1, and a thickness of only 2 μm can absorb 99% light of which energy is larger than the energy gap of CdSe. During manufacturing process, when depositing a cadmium telluride thin film, because the vapor pressure of elemental tellurium and the vapor pressure of elemental cadmium are almost the same, the composition of CdTe will be self-stabilizing and thus the proportion of tellurium is very close to the proportion of cadmium. As a result, even though the accuracy of an instrument is not superior, the stoichiometric ratio of tellurium to cadmium of a deposited cadmium telluride thin film is approximately 1. Hence, using CdTe is easier in respect of manufacturing process. Currently, the highest efficiency on laboratory scale is 16.5%. Because cadmium is a heavy metal, which results in heavy metal contamination to the environment, legislation should be made to control the usage of cadmium to avoid damaging the environment.
Group I-III-VI semiconductors are materials derived from group II-VI semiconductors, and CuInSe2 (CIS) is one of the group I-III-VI semiconductors. As shown in FIG. 1 and FIG. 2, CIS has a structure of chalcopyrite, of which lattice parameter a is 5.78 Å and c is 11.62 Å. The structure of chalcopyrite can be regarded as a stack of two hexagonal zincblend structures wherein cations are arranged in an ordered manner along the crystal c axis. The energy gap of CIS is about 1 eV and the absorption coefficient of CIS is 105 cm−1, as mentioned in Firoz Hasan, S. M. et al. Sol. Energy. Mater. Sol. Cells 58, 349 (1999). CIS is a popular direct energy gap material because a thickness of 0.1 μm can absorb most of the incident light. According to the known spectra of varied common-used semiconductors and absorption coefficients thereof at different wavelengths, CIS is known to absorb light of which wavelength covers from UV light to infrared light.
FIG. 3 is a Cu2Se—In2Se3 pseudo binary phase diagram, which indicates that the allowable deviation from stoichiometry (Cu:In:Se=1:1:2) is about 5%, namely, as long as the ratio of Cu to In is between 0.82 and 1.04, CuInSe2 has physical properties and chemical properties of chalcopyrite structure.
Moreover, conductivity type and electrical property can be changed through changing ratio of Cu to In, as mentioned in Bindu, K. et al. Sol. Energy. Mater. Sol. Cells. 79, 67 (2003) and Deepa, K. G. et al. Sol. Energy. 83, 964 (2009). When Cu/In>1 (Cu-rich), the CIS thin film is P-type, of which resistivity is lower, the surface is rougher and the grains are larger. When Cu/In<1 (In-rich), the CIS thin film is N-type, of which resistivity is higher, the surface is smoother and the grains are smaller. A typical structure of CIS solar cell is shown in FIG. 4.
Some researchers discover that adding gallium into CuInSe2 changes the energy gap of the material, leading to the development of CuInxGa1-xSe (CIGS). One of the advantages of CIGS is that an energy gap gradient within the absorber layer can be generated through adjusting the ratio of In to Ga, and the ratio gradually increases in a direction from the substrate to the buffer layer, which assists in absorbing different wavelengths of light, and thus achieving a high light-use efficiency.
Some publications indicate that using sodium-lime glass as substrate instead of silicon substrate has a positive effect, as mentioned in Ruckh, M. et al. Sol. Energy. Mater. Sol. Cells. 41/42, 335 (1996). In general, the reason for said positive effect is that when depositing a CIGS absorber layer, because of heating, a small amount of sodium ion of the substrate diffuses into CIGS, which results in larger grains, better electrical conductivity, reducing serial resistance and increasing open-circuit voltage of the device. However, many problems still remain to be solved. A typical structure of CIGS solar cell is shown in FIG. 5. The highest efficiency on laboratory scale is 19.9% at present, which is achieved by Repins. et al., as mentioned in Repins, I. et al. IEEE Photovoltaics Specialists Conference Record. 33 (2008).
For the purpose of facilitating popularization of solar cells entirely to make solar cells replace most of the energy sources as the main source of power system, the most important issues lie in how to increase conversion efficiency and how to reduce cost. Although the conversion efficiency of the quaternary compounds of copper indium gallium selenide is the highest among the thin-film solar cells, the cost is high because both indium and gallium are precious metals.
In 2011, Prieto et al. at Colorado State University has found that copper selenide is a high-effective and cheap material for solar cells, as mentioned in Riha, S. C. et al. J. Am. Chem. Soc. 133, 1383 (2011). Compared to the costly solar cells based on quaternary compounds of copper indium gallium selenide thin-film, copper selenide without containing precious metals has predominance in price.
In 1998, Prieto et al. has used hydrothermal at 90° C. for 4 hours to successfully synthesize Cu2-xSe, as mentioned in Wang, W. et al. J. Mater. Chem. 8, 2321 (1998), and the mole ratio of CuI to Se they use is 2.2/1. In 2002, Liu et al. has also used hydrothermal at 60° C. for 2 hours to synthesize Cu2-xSe rod-like nanocrystal, as mentioned in Liu, Y. et al. Mater. Res. Bull. 37, 2509 (2002), and the mole ratio of CuSO4 to Na2SeSO3 they use is 0.75/1. In 2002, Xie et al. has utilized ultrasonic irradiation both at a ratio of Cu to Se of 1/1 and 1/2 respectively to synthesize Cu2-xSe, as mentioned in Xie, Y. et al. Inorg. Chem. 41, 387 (2002). In 2010, Zhang et al. has used pyrolysis at 250° C. for 5 minutes at a ratio of Cu to Se of 2/1 and 1/1 respectively to synthesize Cu2-xSe, as mentioned in Zhang, A. et al. Mater. Chem. Phys. 124, 916 (2010).
From the conventional methods noted above, to synthesize Cu2-xSe nano crystal, the key point is not the ratio of copper to selenium but is the reaction conditions such as reaction temperature, reaction time and the used solvent. However, some publications state that an additional crystalline phase will be formed if excessive amount of selenium participates in the reaction. Grozdanov uses a ratio of Cu to Se of 1/1 and of 1/4 respectively under the same reaction conditions and Grozdanov finds that Cu2-xSe is formed when the ratio of Cu to Se is 1/1 while CuSe is formed when the ratio of Cu to Se is 1/4, as mentioned in Grozdanov I. Synth. Met. 63, 213 (1994). CuSe is also a selenide semiconductor and the energy gap of CuSe is 3 eV, with reference to Pejova, B. et al. J. Solid State Chem. 158, 49 (2001).
In 1997, Clement et al. has proposed that when x is larger than 0.2, the Cu3Se2 crystalline phase will exist in Cu2-xSe crystalline phase and acts as an impurity phase, resulting in worsening the properties of Cu2-xSe, as mentioned in Clement C. L. et al. Thin Solid Films 302, 12 (1997). Cu3Se2 (copper(II) selenide) is a binary semiconductor compound whose energy gap is about 2.6 to 2.8 eV and whose resistivity is about 10−3, as mentioned in Yan, Y. L. et al. Inorg. Chem. Commun. 6, 34 (2003). Compared to Cu2-xSe, because Cu3Se2 has a worse absorption coefficient, the applicability of Cu3Se2 is lower. Dhanam et al. also mentions that when x is smaller than 0.2, Cu2-xSe is more stable, as mentioned in Dhanam, M. et al. J. Cryst. Growth 280, 425 (2005). In conclusion, to synthesize Cu2-xSe, the ideal value of 2−x is 2.00≧2−x≧1.80.
In respect of copper selenide solar cells, in 1984, Stewart et al. has fabricated a copper selenide solar cell of which conversion efficiency is 5.4% through using glass as substrate, evaporating CdS as N-type and buffer layer respectively, evaporating Cu2-xSe as P-type, and using Ag as back electrode and upper electrode, as mentioned in Chen, W. S. et al. Appl. Phys. Lett. 46, 1095 (1985). So far the structure with the highest conversion efficiency revealed by publication is through using silicon as N-type, using Au/Sb alloy as electrodes, and evaporating Cu2-xSe as P-type, and the conversion efficiency is 8.8%, as mentioned in Okimura, H. et al. Thin Solid Films 71, 53 (1980). Moreover, if an anti-reflection layer is added to the structure, the anticipated efficiency will reach over 10%.
Although copper selenide, which is a Group I-VI semiconductor and belongs to binary compounds, is a promising material for solar cells, the conventional method for synthesizing copper selenide nanocrystal is by means such as hydrothermal, ultrasonic irradiation pyrolysis and so forth, hence, the problem of high production cost still remains.
In addition, conventional methods for fabricating copper selenide thin films, such as spraying, printing, sputtering and so forth, need to be operated under high temperature environment to allow sintering process to proceed in order to remove binder, or such conventional methods need to be performed under vacuum conditions to allow coating process to proceed, not only the manufacturing processes are not convenient but also related equipments, maintenance and operating cost are extremely costly.
To overcome the shortcomings, the present invention provides a method for making Cu2-xSe nanoparticles and method for making a deposited Cu2-xSe thin film by electrophoresis to mitigate or obviate the aforementioned problems.